The presence of the endothelial layer reduces nitric oxide-induced hyporesponsiveness to phenylephrine in rat aorta

A single exposure to nitric oxide (NO) donors produces a long-lasting hyporesponsiveness to phenylephrine (HRP) in rat aorta rings. Here the authors investigate the role of the endothelial layer in the development of NO-induced HRP and the putative role of endothelium-derived vasoconstrictors in counteracting it. The NO donor S-nitrosoacetyl-D,L-penicillamine (SNAP) induced a dose-dependent reduction in the maximal effect (E.,,,) of phenylephrine. In rings without endothelium, E-max dropped to 60 %,25 %, and 10% of control values 1 h after a 30-min incubation with SNAP (2, 20, and 200 muM, respectively). In contrast, the presence of endothelium prevented the HRP induced by 2 muM SNAP and significantly reduced the HRP elicited by 20 and 200 muM SNAP (E-max reductions of 50% and 65%, respectively), thereby characterizing the endothelium protective effect. Superoxide dismutase (SOD; 100 IU/mL), MnTBAP (a nonenzymatic SOD mimetic; 100 muM), captopril (10 muM), MK886 (a lipoxygenase inhibitor; 10 muM) and BQ 123 (endothelin receptor A antagonist; 1 muM) did not change the endothelium protective effect. Therefore, increased release of vasoconstrictors that would counteract NO-induced loss in phenylephrine responses cannot account for the protective effect of endothelium. In contrast, oxidation of sulphydryls with DTNB prevented the onset of SNAP-induced HRP. A better understanding of mechanisms by which the endothelial layer (or protein sulphydryl groups present in it) exerts its protective effect towards the NO-induced loss in physiological vasoconstriction is likely to be of value in cardiovascular conditions such as ischemia/reperfusion and septic shock.